The invention relates generally to a satellite-switched time-division multiple access (SS-TDMA) system. More specifically, the invention relates to a circuit for establishing and detecting the duration of received bursts and the guard space therebetween in a satellite-switched time-division multiple access system.
A typical satellite-switched time-division multiple access system is shown schematically in the view of FIG. 1. The system includes plural transmitting earth terminals 10 and plural receiving earth terminals 14 and a relay satellite 12 for receiving, switching and subsequently retransmitting the information burst signals received from the transmitting earth terminals 10. In such a system, each of the transmitting earth terminals 10 can communicate through the satellite 12 with a specific one of the receiving earth terminals 14. In FIG. 1, the data bursts transmitted by the various transmitting earth terminals 10 and retransmitted by the satellite 12 are identified by one of a solid rectangle, a hatched rectangle and an open rectangle to identify the receiving terminal for which the bursts are intended. Data is transmitted from each transmitting earth terminal in a series of frames each of which can contain bursts of data intended for any one of the receiving earth terminals 14.
The satellite 12 must identify to which of the receiving earth terminals 14 a particular burst of received data is intended and then route the burst of data through a switching circuit to on-board transmitting facilities which direct the burst of data onto an appropriate beam to the intended receiving earth terminal 14.
A predetermined minimum amount of time, termed a "guard space" or "guard time" is required between bursts to prevent interference between earth terminals and to permit the satellite to perform the necessary switching operations for properly routing the data bursts. If the guard time is too short, due to differences in propagation delays and other factors, two separate data bursts may overlap one another at a receiving earth terminal 14 and thus interfere with one another causing a loss of transmitted information. It is thus necessary to monitor the amount of guard time which is being provided by the transmitting earth terminals 10 and by the satellite 12 in order to ensure that the system is operating properly and that there will be no interference between data bursts retransmitted by the satellite 12.
In performing the monitoring function, it is also desirable to provide a signal which indicates the presence of received data bursts. Such a signal is useful in determining whether or not the data bursts are of the correct length and for activating and synchronizing the digital processing circuitry which receives the data bursts. It is obviously desirable that a guard space signal and a burst duration signal which are produced indicative of the guard space and burst times, respectively, be aligned as closely as possible with the appropriate time periods in the received signal envelope to minimize the measuring error and to maintain a minimum guard space.
Prior art proposals for generating burst duration and guard space signals did not produce entirely satisfactory and accurate results. The primary reason for this failure relates to the type of modulation employed and problems inherent therewith. Specifically, in nearly all satellite communication systems, the well-known QPSK (Quaternary Phase Shift Keying) modulation technique has been employed. According to this technique, an incoming bit stream is divided into two channels which are then used to modulate in-phase and out-phase carriers which are subsequently recombined prior to transmission. With this technique, each symbol period bears information relating to two bits of the input data stream.
FIG. 2 is a diagram which illustrates transitions between the four possible states of the transmitted signal produced according to the QPSK technique. In this diagram, P and Q denote, respectively, the two digital channels into which the incoming bit stream is divided. There are four possible phase states of the modulated carrier corresponding to input data stream bit combinations of 0,0; 0,1; 1,0 and 1,1. For instance, for transitions between 0,0 and 0,1 and between 0,0 and 1,1 the path traversed during the transition does not pass near or through the origin and hence the amplitude of the transmitted signal does not pass through or near 0. On the other hand, for a transition between 0,0 and 1,1 or between 0,1 and 1,1 ideally, the path is through the origin. Because of various nonlinearities and non-ideal components, the path may not pass precisely through the origin. Nevertheless, the path during the transition passes near the original so that the amplitude of the output transmitted signal undergoes a sharp dip or near null during certain phase transition times.
This is illustrated further by the diagram of FIG. 3 which shows a typical transmitted waveform for a particular TDMA burst containing a number of near nulls due to 180 degrees phase transitions passing near the origin as described above.
In prior approaches to the generation of the burst duration and guard space signals, the received signal was envelope-detected and the amplitude of the extracted envelope signal compared with a fixed reference value. The results of the comparison were taken as the burst duration signal and the inverse of the burst duration signal thus produced as the guard space signal. Unfortunately, this technique was not successful because the near nulls produced a false burst duration signal due to the magnitude of the extracted envelope falling below the fixed reference value.
This technique could not easily be improved upon. For instance, if the extracted envelope of the bursts were subjected to low-pass filtering in order to smooth out the near nulls so that the signal did not contain portions which fell below the reference value, the accuracy of the burst duration signal and guard space signal with respect to the actual timing of the burst was lowered. Similarly, filtering the comparison signal would also produce inaccuracies in the alignment of the burst duration signal and guard space signal with the actual timing of the received burst. Moreover, it was not practical with such a technique to lower the reference value significantly because of the possibility of producing false burst duration and guard space signals due to the presence of noise. Yet further, the presence of a large number of near nulls in close succession would nearly always produce a false burst duration signal.